Land or Air travel? I think there may be two different guidelines.
You are right, there are two different dates of implementation. Found the following info:
Q. When will the travel initiative be implemented?
In the proposed implementation plan, the initiative will be implemented in two phases. The proposed timeline will be as follows:
January 23, 2007 – Passports, Merchant Mariner Documents (MMDs) or NEXUS Air cards would be required for all air travel from within the Western Hemisphere for citizens of the United States, Canada, Mexico, and Bermuda.
January 1, 2008 – It is anticipated that on January 1, 2008, U.S. citizens traveling between the U.S. and Canada, Mexico, Central and South America, the Caribbean, and Bermuda by land or sea (including ferries), may be required to present a valid U.S. passport or other documents as determined by the Department of Homeland Security. While recent legislative changes may permit a later deadline, both the Departments of State and Homeland Security are working to put all requirements in place by the original deadline. Advance notice will be provided to enable the public meet the land/sea border requirement.
Q. Originally, the proposed date of implementation for the air phase was January 1, 2007. Why the change?
The Departments of Homeland Security and State proposed a later date in response to comments from the airline industry in order to help facilitate holiday travel. The requirement will go into effect on January 23, 2007 , 60 days after publication of the Final Rule in the Federal Register.